WHILE ENGAGED IN SEISMIL WORK, THE PLT WAS CONTACTED BY RADIO & WAS ASKED TO ASSIST IN FREEING 2 CREW TRUCKS THAT HAD GOTTEN STUCK IN SAND DUNES. HE DECIDED TO ASSIST SINCE THE STUCK TRUCKS WERE DELAYING THE ENTIRE OPERATION. HE PLANNED TO PARTIALLY RAISE & PULL ONE END OF A TRUCK BY USING A 100 FT LONG LINE. THE HELICOPTER WAS FACING THE AFT END OF THE TRUCK & LONG LINE WAS AT ABOUT A 25-30 DEG ANGLE WHEN CONTINUOUS COLLECTIVE POWER WAS APPLIED. BY THE PLT. THE HELICOPTER THEN YAWED LEFT. THE PLT ATTEMPTED TO COMPENSATE WITH RIGHT ANTI-TORQUE PEDAL, REDUCED POWER & INDUCED RIGHT FORWARD CYCLIC. REPORTEDLY, THE HELICOPTER THEN TURNED 360 DEG & CAME TO REST ON ITS RIGHT SIDE. AT SOME POINT AFTER THE LEFT YAW, THE PLT RELEASED THE LONG LINE. THE COMPANY SAFETY OFFICER REPORTED THAT IF A 150 FT LONG LINE HAD BEEN USED, THERE WOULD HAVE BEEN A LARGER MARGIN FOR ERROR.